Sorry Galahad, but "Unregistered" is much more correct than you are. Don't want to make you down, that's just a fact.

First of all, I'm sure that 100% of all Amiga games use the blitter.

But even if you COULD handle blitter operations with the Dreamcast's GFX processor, this wouldn't work well in an emu, because you have to synchronize the cpu and blitter. If the GFX processor would do blitter stuff at the same time the DC cpu is emulating the Amiga cpu, the whole emulation would fuck up.

Also, I don't even think you could handle blitter operations with the DC's GPU. The blitter uses a very different graphics format than the DC, and you'd have to convert the graphics, probably more than one time, because the blitter can do lots of different stuff with the data it blits to make it look different. It's almost impossible to handle all the different things the blitter can do.
(It would take too long to explain everything.)

Another big problem is the line-by-line system. If a game doesn't use this, it would be OK, and you could easily display a playfield (after a graphics conversion). But what are you going to do, if the copper (or even the processor) changes the scroll variables during different lines of the display? Or even the modulo registers? Or displaying another playfield in another resolution and color depth? And what are you doing with colour changes ("copper rainbows")?

About the SNES (and most other game consoles):
The way it operates is almost completly different from how PCs operate. Many of them only have a very slow processor. That's not a problem to emulate. But the graphics chip is, as it's working completly different than you would probably expect. And like the Amiga, it uses a line-by-line display, too, so you can't just display the whole picture, you have to build it up line by line, and that's definitly not, what a DC's GPU is supposed to do.

(By the way, buy a PC with an outdated Kyro2 and you got yourself a Dreamcast but with a faster CPU)

@jmmijo:
Two reasons not to use Assembler anymore:
-Todays games and apps are too big to be handled in Assembler
-Todays C compilers usually produce faster code than one could write in Assembler (what optimizing compilers do would turn your assembler prog into a big piece of chaos)